A visible image is formed in silver halide photographic materials by exposure of the material to actinic radiation to form a record of the exposure which is invisible to the unaided eye, followed by processing of the material to yield a visible image.
The invisible record of exposure is referred to as a latent image. It is generally agreed that the latent image comprises minute specks of metallic silver formed in or on individual silver halide grains by interaction between silver ions and photoelectrons generated by absorption of actinic radiation by the silver halide grains.
Processing of most common silver halide photographic materials includes a development step in which the material is contacted with an aqueous alkaline solution of a developing agent. The developing agent is a reducing agent which will selectively reduce to metallic silver those silver halide grains containing a latent image.
It is known that the latent image is not permanent and that, with the passage of time, silver halide grains which would be developable immediately after exposure become nondevelopable. This phenomenon is termed latent image fading and manifests itself as a loss in image density in the developed image and a consequent loss in speed in the silver halide photographic material.
If silver halide materials were developed immediately following imagewise exposure, latent image fading would not be a problem. However, with many silver halide materials delays between exposure and processing frequently occur. For example, with amateur film materials in which multiple images are formed on a single roll of film there is often a delay of months between the time the first image is exposed and the time the exposed roll of film is sent for processing. With such materials latent image fading can present a significant problem and compounds are added to photographic materials to prevent or reduce it. These compounds are referred to as latent image stabilizing compounds or latent image stabilizers and the prevention or reduction of latent image fading is referred to as latent image stabilization.
Another way to which the developed image can be adversely affected is through a phenomenon known as fogging. Fogging is a result of spontaneous development of unexposed silver halide grains. The grains can be rendered developable during storage, either prior to or subsequent to exposure, or during development itself. In order to minimize this spontaneous development, compounds known as antifoggants are added to the silver halide material, to the developer solution, or to both.
Some compounds used as antifoggants are structurally similar to compounds used as latent image stabilizers. However, it is important to recognize that the two types of compounds are employed for different purposes to obtain different effects. Latent image fading is the loss of developable silver halide grains and results in a loss in density in the developed silver image, while fogging is the development of unexposed silver halide grains and results in an increase in minimum density. Thus, compounds which are known to be useful antifoggants are not necessarily useful as latent image stabilizers, and vice versa.
Among the latent image stabilizers known in the art are the N-alkenyl benzothiazolium and naphthothiazolium salts described in Arai et al. U.S. Pat. No. 3,954,478. In A. H. Herz U.S. Ser No. 236,360, filed Feb. 20, 1981, now U.S. Pat. No. 4,374,196 commonly assigned, acyclic compounds (optionally containing an aryl substituent), which can be obtained by basic hydrolysis of N-alkenyl thiazolium salts, are disclosed to be effective latent image stabilizers.